Safeties step up in final preseason game

Defensive coordinator Vic Fangio has been waiting for a backup safety to catch his eye during the preseason.

The wait might have finally ended Thursday night.

In a four-minute span in the first quarter, three backup safeties – C.J. Spillman, Trenton Robinson and Darcel McBath – took turns in the spotlight against the Chargers’ first-stringers.

First, Spillman stripped San Diego tight end Antonio Gates at the end of an 11-yard reception and Robinson picked up the loose ball and returned it 22 yards to the Chargers’ 32-yard line. Two plays later, Colin Kaepernick tossed a 32-yard touchdown pass to Delanie Walker.

On San Diego’s next drive, McBath read Philip Rivers‘ eyes, stepped in front of receiver Malcom Floyd and returned an interception 19 yards.

Spillman will probably open the season as the No. 3 safety. McBath, Robinson and undrafted rookie Michael Thomas have been battling for a roster spot.

The 49ers will trim their roster from 75 players to 53 by 6 p.m. Friday.

Most starters sit: Alex Smith started at quarterback, but left after the 49ers began their game-opening, 11-play touchdown drive with five straight runs.

Running back Frank Gore, wide receivers Michael Crabtree and Randy Moss, and the entire first-string defense did not play.

Tukuafu shines: Jim Harbaugh said two-way player Will Tukuafu had “one of the best isolation blocks I’ve ever seen” in his role as a fullback. Tukuafu, 6-foot-4 and 293 pounds, overwhelmed Chargers linebacker James Mouton to clear the way for Anthony Dixon on a 6-yard run in the third quarter.

“I could hear it more than I could see it,” Harbaugh said. “I saw it, but it was more impressive what I heard.”